1. Field of the Invention
The present invention relates to a method and related apparatus for acquiring information of a point-to-multipoint (p-t-m) Multimedia Broadcast Multicast Service (MBMS) service in a wireless communications system, and more particularly, to a method and related apparatus for acquiring information of a p-t-m MBMS service for a receiving terminal in a wireless communications system for saving power and resources of the receiving terminal.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates. To enhance multimedia functions in the 3G mobile telecommunications system, a protocol specification developed by the 3rd Generation Partnership Project (3GPP) provides Multimedia Broadcast Multicast Service (MBMS). MBMS is a point-to-multipoint bearer service and is established on an existing network architecture of the Universal Mobile Telecommunications System (UMTS), utilizing Internet Protocol (IP) packets as a medium. Thus, MBMS allows a single source entity to transmit data to multiple user entities simultaneously.
Since the 3GPP has thoroughly defined operations and architecture of MBMS in the related specifications, the operations and architecture of MBMS are described only briefly hereinafter. For the universal mobile telecommunications system (UMTS), the 3G communications system comprises User Equipment (UE), the UMTS Terrestrial Radio Access Network (UTRAN), and the Core Network (CN). Communications protocols utilized include Access Stratum (AS) and Non-Access Stratum (NAS). AS comprises various sub-layers for different functions, including Radio Resource Control (RRC), Radio Link Control (RLC), Media Access Control (MAC), Packet Data Convergence Protocol (PDCP), and Broadcast/Multicast Control (BMC). The sub-layers mentioned, and their operating principles, are well known in the art, and detailed description thereof is omitted. For MBMS, the Core Network further comprises the Broadcast Multicast Service Center (BM-SC) that is not only a newly added network node but also an entry point for receiving external data, e.g. data provided by a content provider. The BM-SC manages provision, delivery, announcement, authorization, and storage of related parameters for MBMS services, and transmits corresponding information to the Gateway GPRS Support Node (GGSN). In the MBMS system architecture, the GGSN plays a role as an entry point for IP multicast traffic, and routes MBMS data to the Serving GPRS Support Node (SGSN) via a proper GPRS Tunnel (GTP). More than one SGSN can exist under the GGSN. The SGSN performs MBMS Bearer Service control functions to accurately transmit data packets to the UTRAN, allowing multiple UEs, which order the same MBMS service in the same UTRAN, to receive the same MBMS data. The SGSN can comprise more than one Radio Network Controller (RNC). The RNCs are responsible for efficiently allocating radio resources for the UEs according to the number of UEs counted by the Node-Bs (NB). The RNC comprises more than one NB that is responsible for transmitting or receiving radio waves to or from the UEs that can be mobile phones or PDAs.
According to the protocol specification developed by the 3GPP, MBMS offers two service modes: Broadcast mode and Multicast mode. When interested in a specific MBMS service, the UE needs to order the BM-SC for the MBMS service by establishing a service agreement including specific service provision phases. These two service modes have different service provision phases. The service provision phase of Broadcast mode includes Service announcement, Session Start, MBMS notification, Data Transfer and Session Stop. The service provision phase of Multicast mode includes Subscription, Service announcement, Joining, Session Start, MBMS notification, Data Transfer, Session Stop and Leaving. For realizing customized services, Joining and Leaving sessions utilized in Multicast mode enable the BM-SC to execute authorization and payment recording for the UEs at the joining session. The service provision phases mentioned above are well known in the art, and will not be described in detail.
From the standpoint of the RRC, all logical data communication exchange channels, be they for providing data transmission exchange to the UE or for providing RRC layer control signal transmission exchange, are defined in the context of a Radio Bearer (RB). In the UE end, the RB comprises one unidirectional, or a pair of, uplink/downlink logic data transmission exchange channels. In the network end, the RB comprises one unidirectional, or a pair of, uplink/downlink logic data transmission exchange channels.
When providing the MBMS service, the UTRAN may obtain the number of the UEs ordering the MBMS service via a counting procedure, and can adopt either one of the following two transmission modes: point-to-point (p-t-p) transmission and point-to-multipoint (p-t-m) transmission. In the p-t-p transmission, transmission between the UEs and the UTRAN utilizes a dedicated-channel, and thereby the Node-B establishes one dedicated channel for one UE to transmit data and configuration. Compared with the p-t-p transmission, the p-t-m transmission allows the CN to provide the data and the configuration for multiple UEs in the same cell simultaneously. In addition, when the MBMS service is in Broadcast mode, only the p-t-m transmission is used. When the MBMS service is in Multicast mode, the UTRAN switches between the p-t-p and p-t-m transmissions according to the number of UEs. In the protocol specification developed by the 3GPP, three logical channels are defined to transmit corresponding MBMS information: MBMS point-to-multipoint Control Channel (MCCH), MBMS point-to-multipoint Traffic Channel (MTCH), and MBMS point-to-multipoint Scheduling Channel (MSCH). The three logical channels are mapped onto a transport channel, Forward Access Channel (FACH), and the FACH is mapped onto a physical channel, Secondary Common Control Physical Channel (S-CCPCH). MCCH is utilized to transmit a corresponding MBMS configuration, including MBMS Radio Bearer Information, MBMS Service Information, and other information. In general, before obtaining the MBMS configuration, the UE first has to receive System Information block type 5 or 5bis through S-CCPCH, and detect whether an MCCH configuration carrying the above-mentioned information is included in the System Information block type 5 or 5bis.
In order for the UE to understand the current status of each MBMS service, the protocol specification of 3GPP TS 25.331 V6.8.0 RRC Release 6 defines a variable, MBMS_ACTIVATED_SERVICES, used for storing information of the MBMS Multicast services the UE has joined, as well as the MBMS Broadcast services the UE is interested in receiving. The stored information in the MBMS_ACTIVATED_SERVICES variable includes Activated service list, Service type, etc. The content of the variable, or the stored information, is updated by the UE as the MBMS service status changes, e.g. when the UE joins or leaves a Multicast service. Additionally, the RRC protocol specification further defines several situations in which the UE is required to perform an MCCH acquisition procedure requesting the base station to transmit MBMS service information and configuration to the UEs. The MBMS service information and configuration includes information related to radio bearers, settings, and preferred frequency of the MBMS services, such as MBMS Modified Services Information and MBMS Common P-T-M RB Information. The UE can determine the current status of the MBMS services provided in the cell based on the above-mentioned information, and thereby can activate the service reception, or modify the corresponding settings. According to subclause 8.7.2 of the 3GPP protocol specification TS 25.331 V6.8.0 RRC, the situations in which the UE is required to perform the MCCH acquisition procedure include:                1. Upon selecting or re-selecting a cell supporting MBMS services, such as during a power-on period of the UE.        2. Upon change of a cell controlling the MBMS service being received by the UE due to an active set update or hard-handover.        3. Upon entering the UTRAN from another radio access technology (RAT), such as Global System for Mobile Communication (GSM).        4. Upon releasing an MBMS point-to-point radio bearer, such as when transiting from p-t-p transmission to p-t-m transmission in an MBMS service.        5. Upon returning from loss of coverage, such as transiting out of a basement or an elevator, to effective receiving coverage.        6. Upon receiving an indication from upper layers that the MBMS activated service has changed, such as when a new MBMS service item is added into the service set.        
According to the protocol specification, the UE applies the MCCH acquisition procedure to determine MBMS services available in the cell and to initiate reception of the MBMS services that the UE has joined. Additionally, the MCCH acquisition procedure is applied to the all UEs supporting MBMS, irrespective of an RRC state of the UEs, such as idle, URA_PCH, CELL_PCH, CELL_FACH and CELL_DCH state. The above-mentioned RRC states are well known in the ordinary art and the details thereof are omitted. Therefore, in the prior art, the UEs not using MBMS service functions or not interested in the current MBMS services still apply the MCCH acquisition procedure, thereby resulting in system resource waste and power consumption.